Some metal waste or other waste may be stored for long periods in appropriate containers due to a dangerous, contaminated or toxic fraction contained in them. Preliminary melting of waste is advantageous in order to reduce their volume. It also distributes the contaminants that are more reducible than the metal throughout the volume of the ingot that is finally obtained, and transfers contaminants that are more oxidisable than the metal in the slag with appropriate composition, if possible an oxide-based slag. The result obtained is thus good confinement properties by reducing the free surface of the metal exposed to corrosion and thus the risks of future oxidation of dangerous elements.
There are many types of melting furnaces. Induction heating is most frequently used, and a field coil is then arranged around a crucible inside which waste is poured. The field coil produces high intensity electrical currents in the waste, heating being sufficient to cause melting. The crucible often needs to be cooled by permanent circulation of a cold fluid, normally water, contained in ducts adjacent to the crucible or formed in the crucible. One example of such a type of induction furnace is described in patent FR-2 835 620-A.
The designer finds himself faced with various difficulties. Firstly, severe thermal, chemical and mechanical forces are applied to the crucible, generated by the heat from the molten bath, its corrosive properties and differential expansions due to the small distance between the molten bath and the cooling circuit. The inside face of the crucible that is most exposed to damage because it is immersed in the molten bath is often composed of a ceramic refractory coating, the remainder of the crucible being metallic, but risks of separation between the metal and the ceramic or crumbling of the ceramic remain since the differential expansions are particularly high at their interface. Protection against breakage of the crucible is necessary, which would cause a mix between the molten bath and the cooling water. Forces applied to the crucible can cause premature wear or even breakage, followed by a dangerous mix between the molten bath and the cooling water.
Protection is also necessary against excessive consumption of induction energy due to parasite currents generated in the wall of the crucible. This is often overcome by breaking the crucible down into sectors, in other words dividing it into angular sectors separated by isolating joints, but additional difficulties then appear at the internal face of the crucible where these joints are exposed to the molten bath and problems arising due to differential expansions are exacerbated by the increase in the number of interfaces between different materials. Such a design is disclosed in WO-03/067 166-A, in which sectors are coated with ceramic, although not all the problems encountered are solved.
Other difficulties appear in the process. The molten metal may be poured into an ingot mould at the end of melting, or progressively withdrawn in the form of an ingot during this process. These first processes can lead to an unwanted mix between the metal and its slag during the pour, while the second processes that are based on the use of cold crucibles involve solidification of the metal close to the ingot drawing off orifice and therefore the molten metal and the cooling water must be very close to each other and consequently lead to increased dangers of accidents due to failure of the crucible. In all processes, the installation has to be made leak tight to prevent dissemination of vapours produced due to melting metal.